This application is related to co-pending U.S. Pat. application Ser. No. 09/713,541 entitled xe2x80x9cSWITCHING POWER SUPPLY PACKAGESxe2x80x9d], filed the same date herewith; Ser. No. 09/585,928, filed Jun. 2, 2000; Ser. No. 09/540,058, filed Mar. 31, 2000; and Ser. No. 09/444,032, filed Nov. 19, 2000, each of which is fully incorporated herein by reference.
This invention pertains generally to the field of power conversion and, more particularly, to switching power supplies with feedback control.
Compact and efficient power supplies are an increasing concern to users and manufacturers of electronics. Switching power supplies with pulse width modulated (xe2x80x9cPWMxe2x80x9d) controllers offer both compactness and efficiency in a number of different topologies. Boost and buck switching power supply topologies are efficient, but do not isolate the power input from the power output. Other topologies, such as the flyback, do isolate the power input from the power output by using a transformer. In such topologies, feedback from the secondary (power output) side of the transformer is needed to adjust the pulse width modulation duty cycle of the power switch
Prior art PWM controlled isolated power supplies typically regulate the output voltage by directly sensing the output voltage, with feedback to the PWM controller via an isolation device (typically a feedback transformer or opto-isolator). This secondary feedback to the PWM controller sets the pulse widths of the power switch in order to maintain proper output regulation. In particular, secondary feedback requires additional components and related costs to the power supply, and represent design complexity related to loop performance and stability.
For example, U.S. Pat. No. 5,313,381 (thexe2x80x9c""381 patentxe2x80x9d) discloses a three-terminal switching power supply control chip for use with a flyback converter. FIG. 1 illustrates a flyback converter 20 according to the ""381 patent. The converter 20 employs a three-pin control chip 22 to supply current from a rectified DC source (Vbb) 28 across an isolating transformer 24 to supply power for a load 26. The power supply chip 22 includes a first terminal 30 coupled to a primary winding 32 of the transformer 24, a second (xe2x80x9cgroundxe2x80x9d) terminal 36 coupled to a primary side ground reference, and a third terminal 40 for accepting a combined feedback control signal (IFB) and a bias supply voltage (Vcc) to operate the control chip 22.
Within the power supply chip 22, the first terminal 30 is alternately coupled to the ground terminal 36 by a power transistor switch 42. PWM control circuitry 44 drives the power switch 42 at a variable duty cycle. When the power switch 42 is ON, current flows through the primary winding 32 and energy is stored in the magnetic core 45 of the transformer 24. When the switch 42 is OFF, a secondary diode 46 is forward biased and the stored energy in the transformer core 45 is released through a secondary winding 48 to a filter/storage capacitor 47 and the load 26. After the transformer 24 is reset, the ON/OFF cycle is repeated.
An error amplifier 50 compares the output voltage Vout across the load 26 with a reference voltage to generate the feedback control signal IFB. The bias supply voltage Vcc is supplied from an auxiliary secondary winding 52 of the transformer 24. The bias supply voltage Vcc is modulated with the feedback control signal IFB in an opto-isolator 54 to create the combined bias voltage, feedback signal Vcc/IFB. A feedback extraction circuit (not shown) in the chip 22 separates the feedback signal IFB from the bias voltage Vcc by sensing the excess current flowing through a shunt regulator. The extracted feedback signal IFB is used to control the output of the PWM circuitry 44 to constantly adjust the duty cycle of the power switch 42 so as to transfer greater or lesser current to the secondary.
On the one hand, to properly compensate the PWM controller based on feedback from the secondary requires extra components and often involves expensive re-design, depending upon the particular application. Yet, prior art isolated power supplies that used feedback only from the primary side of the transformer do not properly account for power losses encountered on the secondary side of the transformer. For example, there are prior art PWM controlled isolated power supplies that used feedback only from the primary side of the transformer by deriving the output voltage by sensing input voltage (VIN) divided by the turns ratio of the power transformer (NP/NS). However, this does not take into account transformer core and copper losses, PCB losses and losses attributed to other secondary components. These losses are represented in aggregate as ZESR. When load current is light, output regulation can be achieved, as follows:
ILOADxcx9c0A,
then
xe2x80x83VESRxcx9c0V,
and
VIN*(NS/Np)xcx9cVOUT(SENSE)xcx9cVOUT(LOAD)
However, as the output load increases, the losses associated with ZESR are not taken into account in the feedback, negatively impacting output regulation, i.e., when:
ILOADxe2x89xa00A,
then
VESRxe2x89xa00V,
and
                              V          IN          *                ⁡                  (                                    N              S                        /                          N              P                                )                    ~              V                  OUT          ⁡                      (            SENSE            )                                =          V              OUT        ⁡                  (          LOAD          )                      ,
because
VOUT(LOAD)=VOUT(SENSE)xe2x88x92VESR
Because in PWM switching power supplies, incremental losses associated with ZESR are not linear with changes in load, it is difficult to compensate for these losses over the entire load range. Although this methodology results in a simple and inexpensive feedback circuit, it is unable to meet the load regulation requirements of many end product applications.
For example, U.S. Pat. No. 5,982,644, (the ""644 patent) discloses a pulse-width-modulated boost converter coupled to a high voltage converter, which in turn is coupled to the primary side of a transformer. The modulation of the boost converter is adjusted according to an amplified error signal representing the difference between the boost converter""s output voltage and the voltage from a current sensing circuit sensing the current through the primary winding. This error signal has no way of sensing and accounting for the losses on the secondary side of the transformer. Thus, the ""644 patent power supply employs a linear regulator on the secondary side of the transformer to maintain a constant voltage over the load. Although this power supply avoids the use of feedback from the secondary side of the transformer, it introduces the expense and loss associated with installing an additional regulator at the load.
Thus, it would be desirable to provide power supply packages for accurately controlling isolated power converter topologies, without requiring feedback from the secondary side of the transformer, thereby easing design and reducing the component count. In particular, such power supplies should compensate for secondary VESR losses over a full range of operating load conditions, thereby reducing design complexity and component cost, and providing improved output load regulation.
In accordance with the present invention, switching power supply is provided for controlling the delivery of power from a source to a load in a transformer-coupled power converter. The power supply includes a power switch that cycles ON and OFF and couples the source to the load, and further includes control circuitry responsive to an error signal for regulating the output voltage without varying the duty cycle of the power switch.
In a preferred embodiment, the power supply is used to control the delivery of power from a source voltage to a load in a flyback power converter. The control circuitry comprises a pulse generator coupled to the power switch, the pulse generator producing a train of constant frequency, constant ON time switching pulses for driving the power switch. A pulse rate controller responsive to an error signal is coupled between the pulse generator and the power switch. The pulse rate controller thereby regulates an output voltage at the load by controlling the rate of switching pulses delivered to the power switch over time.
When the power switch is ON, current flowing through the primary winding is sensed, and peak detected. The detected peak current for each power switch ON cycle, (encoded as a voltage), is compared to a compensation signal representing an expected power loss between the source and load, e.g., due to transformer winding and diode inefficiencies, represented in aggregate as ZESR, in order to derive an estimate of the source voltage. When the power switch 122 is turned OFF, the drain voltage is sensed after the switching transient, but before the transformer is reset, so as to reflect a xe2x80x9cflyback voltage.xe2x80x9d This flyback voltage is compared to the estimated source voltage to derive an estimate of the output voltage. The estimated output voltage is than scaled and compared to a voltage reference representing a scaled desired output voltage to derive the error signal used for controlling the pulse rate controller.
Other objects and features of the present inventions will become apparent hereinafter.